Method for producing iron-nitride powders

ABSTRACT

A method for producing iron-nitride powder comprises the steps of introducing iron powder and NH 3  gas or N 2  gas in a vessel, and milling the iron powder in the NH 3  gas or the N 2  gas. Furthermore, a method for producing iron-nitride powder comprises the steps of introducing iron powder and intermetallic compound powder of iron and nitrogen in a vessel, and milling the iron powder and the intermetallic compound powder. The obtained iron-nitride powder consists essentially 85.1 to 99 at % of iron and the rest of nitrogen, and has a body centered cubic structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing iron-nitridepowders in which nitrogen atoms are dissolved into α-iron. The resultingiron-nitride powders will be used as an excellent magnetic substance.

2. Description of the Related Art

A metastable intermetallic compound, Fe₁₆ N₂, comprising 16 iron atomsand 2 nitrogen atoms has a unit cell possessing body centered tetragonal(described as bet) crystal structure and exhibits excellent magneticcharacteristics. This intermetallic compound is produced by vapordeposition of Fe in a reduced N₂ gas atmosphere. (See J. Appl. phys., 67(1990) 5126, Appl. phys. Lett., 20 (1972) 492))

Inventors have paid attention to the magnetic characteristics of theintermetallic compound and tried to develop a method for easilyproducing Fe-N alloy having the same magnetic characteristics as that ofthe intermetallic compound.

As for a compound comprising iron and nitrogen, Fe₄ N has been known.The compound has γ'-phase and is obtained by annealing α-Fe attemperatures between 500° C. and 600° C. in a nitrogen gas or NH₃ gasatmosphere. Nitrogen atoms hardly dissolve into α-Fe at room temperatureas described in the Fe-N binary phase diagram shown in FIG. 16. So,iron-nitride having body centered cubic crystal structure cannot beproduced by the annealing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forproducing iron-nitride powder at room temperature by making use ofmechanochemical reaction, thereby obtaining a substance which hasexcellent magnetic characteristics.

It is another object of the present invention to produce iron-nitridepowders with body centered cubic (hereinafter described as bcc) crystalstructure.

Further object of the present invention is to produce: easily theiron-nitride powder with bcc crystal structure.

The present inventors expected that they could obtain the substancehaving excellent magnetic characteristics due to the reaction in whichnitrogen atoms are dissolved into α-Fe. They happened to think of makinguse of mechanochemical reaction, and succeeded in nitrizing α-Fe atalmost room temperature under the condition of α-Fe.

The inventors tried to react α-Fe powder and ammonia gas (hereinafterdescribed as NH₃ gas) filled in a ball mill vessel by means of ballmilling. In this method, nitrogen atoms in NH₃ molecules was absorbedinto α-Fe powder, and iron-nitride powder having bcc crystal structureand hydrogen gas were produced. This reaction may be represented as thefollowing formula:

    2Fe+NH.sub.3 →2(Fex-N)+3H.sub.2

Then, the inventors tried to react α-Fe powder and γ'-Fe₄ N powder bymeans of ball milling. In this solid state reaction, γ'-Fe₄ N havingface centered cubic (hereinafter described as fcc) structure disappearedand iron-nitride powder with bcc structure was produced. This reactionmay be represented as the following formula:

    Fe+Fe.sub.4 N→Fex-N

In the above two cases (i.e. in case of using NH₃ gas and in case ofusing γ'-Fe₄ N), iron-nitride powder with boo crystal structure has beenobtained. Regarding the X-ray diffraction spectra for the obtainedsubstances, as peaks of α-Fe were broaded, the peaks shifted to a lowangle side. When γ'-Fe₄ N powder having fcc structure was used, ;theintensity of diffraction peaks of γ'-Fe₄ N decreased and finallydisappeared as the reaction progressed. So, the obtained substance wouldhave bcc structure.

Coercive force of the obtained Fex-N is superior to that of α-Fe.

Based on the above-mentioned results, the inventors have perfected theinventions for producing iron-nitride powders.

A method for producing iron-nitride powders according to the presentinvention comprises the steps of:

introducing iron powder and NH₃ gas or N₂ gas in a vessel; and

milling the iron powder in the NH₃ gas or the N₂ gas.

Another method for producing iron-nitride powders according to thepresent invention comprises the steps of:

introducing iron powder and intermetallic compound powder iron andnitrogen in a vessel; and

milling the iron powder and the intermetallic compound powder.

the mechanochemical reaction, the magnetic characteristics of theobtained Fex-N was deteriorated by oxygen and carbon. So, it isnecessary to control the contamination of oxygen and carbon.

As for reacting material, NH₃ gas is substituted by reducing nitrogengas such as the mixed gas consisting of nitrogen gas and hydrogen gas,and Fe₄ N is substituted by other intermetallic compound such as Fe₂ N.

The mechanochemical reaction of α-Fe and Fe₄ N or Fe₂ N can be carriedout in an atmosphere of an inert gas, NH₃ gas or the mixed gasconsisting of N₂ and H₂.

The reacting temperature must be low in order that bcc-(Fex-N) may notbe decomposed into α-Fe and Fe₄ N. The desirable reacting temperature islower than 160° C.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of itsadvantages will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings and detailedspecification, all of which forms a part of the disclosure:

FIG. 1 is a X-ray diffraction pattern for pure iron (α-Fe).

FIG. 2 is a X-ray diffraction spectra for iron-nitride powder whichcontains 10 at % of nitrogen.

FIG. 3 is a partial enlarged view of FIG. 2.

FIG. 4 is a X-ray diffraction spectra for iron-nitride powder whichcontains 14.9 at % of nitrogen.

FIG. 5 is a X-ray diffraction spectra for iron-nitride powder whichcontains 22 at % of nitrogen.

FIG. 6 is a partial enlarged view of FIG. 5.

FIG. 7 is a X-ray diffraction pattern for the sample of 22 at % ofnitrogen after annealing at 250° C.

FIG. 8 is a diagram of 2 in FIG. 1 through FIG. 7.

FIG. 9 shows some of crystallographic planes of γ'-Fe₄ N.

FIG. 10 shows a relation of nitrogen content and particle size.

FIG. 11 shows a relation of nitrogen content and saturationmagnetization.

FIG. 12 shows a relation of nitrogen content and coercive force.

FIG. 13 shows X-ray diffraction patterns of the product which wereproduced from powders of γ'-Fe₄ N and pure iron.

FIG. 14 is a view for representing the change of saturationmagnetization of iron-nitride powder in regard to milling time.

FIG. 15 is a view for representing the geometry of X-ray diffractionexperiment.

FIG. 16 is a Fe-N phase diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having generally described the present invention, a furtherunderstanding can be obtained by reference to the specific preferredembodiments which are provided herein for purposes of illustration onlyand are not intendedto limit the scope of the appended claims.

The Preferred Embodiments according to the present invention will behereinafter described with reference FIGS. 1 through 16.

First Preferred Embodiment

In the First Preferred Embodiment, a planetary type ball mill (FlitschP-5)was used. Twenty balls made of bearing steel having a diameter of 10mm andtwenty grams of iron powder (smaller than 200 mesh, 99%) werefilled withinthe ball mill vessel (content volume being 80 cc) made ofdie steel. Then, ammonia gas at a pressure of 1.5 atoms was suppliedinto the vessel.

The ball mill vessel was rotated at 440 rpm. This treatment intended tocause the reaction described as the following formula:

    2Fe+2NH.sub.3 →2[Fe-N]+3H.sub.2

Iron-nitride powder would be produced by this reaction.

It took 50 hours to react a whole amount of ammonia gas in the vesselwith iron powders on condition that the rotation speed is 440 rpm.

FIG. 1 shows X-ray diffraction pattern of pure iron (α-Fe) powder whichis not reacted with nitrogen. In FIG. 1, 2θis twice an incident angle θas described in FIG. 8. FIG. 9 illustrates the crystallographic planesfor cubic unit cells.

FIG. 2 shows X-ray diffraction pattern of a resulting powder sample inwhich the above-mentioned treatment was performed 8 times. As comparedwith pure iron, the structure of the resulting powder is identified asbcc, and the diffraction lines shifted to a small angle side. These datashow that nitrogen atoms have dissolved in α-Fe and expand its lattice.

FIG. 3 shows an enlarged view of the above x-ray diffraction pattern ofFe(110).

The average amount of nitrogen of iron-nitride powder at this stage isabout 10 at %. As nitrogen is hardly dissolved into α-Fe at roomtemperature, it is considered that nitrogen atoms are super-saturatedwithin the iron of bcc crystal structure.

FIG. 4 shows X-ray diffraction pattern of an iron-nitride powder samplewhich includes 14.9 at % of nitrogen. As compared with pure iron, thestructure of the iron-nitride powder is bcc, and the diffraction lineshifted to a small angle side. These data show that nitrogen isdissolved in α-Fe and the lattice constant of α-Fe becomes large.

The iron-nitride powder which included 1.4.9 at % of nitrogen wasannealed at 250° C. for 24 hours. This annealing decomposed theiron-nitrideinto α-Fe and γ'-Fe₄ N, which were stable phases asdescribed in Fe-N phase diagram shown in FIG. 16.

FIG. 5 shows X-ray diffraction pattern of an iron-nitride powder inwhich milling treatment was performed 20 times. The average amount ofnitrogen in this sample was about 22 at %.

γ'-Fe₄ N is a stable phase at this composition as shown in Fe-N phasediagram, but the ε-phase is produced.

FIG. 6 shows an enlarged view of the above X-ray diffraction pattern ofthearea around 2 θ=42° C. Each position of three diffraction linesalmost corresponds to that or ε-Fe₂₋₃ N (100) (002), (101),respectively.

The iron-nitride powder including 22 at % of nitrogen was annealed at400° C. for 24 hours. This annealing changed the sample into γ'-Fe₄ Nand a small amount of unreacted material as shown in FIG. 7.

The milling treatment was performed several times. This is why enoughamount of NH₃ gas necessary to react cannot be supplied into the vesselbecause the volume of the vessel is small. If NH₃ gas is suppliedcontinuously into the vessel, it is unnecessary to perform the millingtreatment several times.

In the First Preferred Embodiment, the balls made of bearing steel andthe ball mill vessel made of die steel were used. In case that balls anda ball mill vessel, both of which; were made of iron, were used, it wasconfirmed that the same reaction between iron powder and ammonia asdescribed in the First Preferred Embodiment occurred.

FIG. 10 shows the size of crystallite measured by Hall Plot on the basisofthe line width of X-ray diffraction profile. It is clear that theamount ofnitrogen increases and the diameter becomes small.

FIG. 11 shows the amount of nitrogen and saturation magnetization. Whentheamount of nitrogen is increased to 19.6%, magnetization becomes smallbecause ε phase is generated.

FIG. 12 shows the relationship between the amount of nitrogen andcoercive force. As the absorption of nitrogen is progressing, coerciveforce is increasing. When the amount of nitrogen is almost 15 at %,coercive force becomes 2.6 times larger than that of α-Fe. Whennitriding further progresses and phase is generated, coercive forcedecreases. As known fromthe result, the iron-nitride in which the amountof nitrogen is 14.9 at % is suitable for hard magnetic material.

When the amount of nitrogen is less than 5 at %, coercive force isalmost same as that of pure iron. When the amount of nitrogen exceeds 20at %, ε phase is generated. So, this unsuitable for magnetic materialwith high coercive force as described in the present invention.

When the amount of nitrogen which is dissolved into iron is larger than0.4at %, saturation magnetization is not deteriorated. When the amountof nitrogen which is dissolved into iron is more than 20%, thesaturation magnetization decreases to half of that of pure iron. So,this is unsuitable for an application as magnetic material.

Second Preferred Embodiment

In the Second Preferred Embodiment, a planetary type ball mill (FlitschP-5) was used. Twenty balls made of bearing steel having a diameter of10 mm was used. A mixture of Fe₄ N powder having fcc structure (smallerthan 200 mesh) and pure iron powder (smaller than 200 mesh) was used.The averaged nitrogen concentration is 11 at %. The mixed powder wasfilled within the ball mill vessel (the volume is 80 cc) made of diesteel. Then,argon gas at a pressure of 0.8 kg/cm₂ was supplied into thevessel. The ball mill vessel was rotated at 440 rpm.

γ'-Fe₄ N powder possesses the perovskite structure and is generated bynitrizing Fe.

FIG. 13 shows X-ray diffraction spectra for Fe-Fe₄ N powders milled forvarious milling time. At 0 hour in the X-ray diffraction chart, peaks ofFe of α phase having bcc structure (α-Fe in FIG. 13) and Fe₄ N of γ'phase (γ'-Fe₄ N in FIG. 13) are clearly observed. As it takes more timefor ball-milling, (for example, 2.5 hours,7 hours, 10 hours, . . . 200hours, 300 hours), the peaks of γ' phasedisappear. Only the peaksassociated with bog structure remains. The peak of Fe shifted to a lowangle side, and lattices of bcc structure are expanded since N atomic isintroduced into the bcc matrix.

FIG. 14 shows the saturation magnetization of the powders obtained byball-milling Fe₄ N powder and Fe powder. As it takes more time formixing Fern and Fe in the ball mill, the saturation magnetizationdecreases from 191 emu/g to 85 emu/g. After the mixed substance has beenkneaded for 100 hours, the saturation magnetization restores to 191emu/g and never decreases.

Saturation magnetization is measured by means of sampling (shown asblack circle in FIG. 14) which is performed 8 times during 100 hours.Though thesampling is performed in a glove box filled with Ar gas, thiscauses slightoxidation due to residual oxygen in the glove box.

The magnetization of a sample milled for 100 hours without sampling isshown as white circle in FIG. 14. The saturation magnetization of thissample is 197 emu/g. Therefore, it is concluded that the powder thusproduced possess larger magnetization than the starting powders.

What is claimed:
 1. A method for producing iron-nitride powder,comprising the steps of:introducing iron powder and an NH₃ gas or N₂ gasatmosphere into a vessel; and milling said iron power in said NH₃ gas orsaid N₂ gas atmosphere at a temperature less than 160° C.
 2. The methodof claim 1, wherein the atmosphere in said vessel is a N₂ gas atmospherefurther containing hydrogen gas.
 3. A method for producing iron-nitridepower, comprising the steps of:introducing iron powder and anintermetallic compound powder of iron and nitrogen into a vessel; andmilling said iron powder and said intermetallic compound powder under aninert gas atmosphere or an ammonia gas or nitrogen atmosphere or a mixedgas atmosphere of N₂ and H₂ at a temperature less than 160° C.
 4. Themethod of claim 3, wherein said intermetallic compound is Fe₄ N.
 5. Themethod of claim 3, wherein said intermetallic compound is Fe₂ N.
 6. Themethod of claim 3, wherein the atmosphere in said vessel is an inert gasatmosphere.
 7. The method of claim 3, wherein the atmosphere in saidvessel is ammonia or nitrogen.